The real answer is to use mount options.
ADFS, AFFS, FAT (and variants), HFS, HPFS, ISO9660, and UDF all support the 'gid' option. Combine that with the 'users' option, and anyone could mount and access files as long as they were in the right group (floppy, plugdev and disk come to mind.
As for doing this automatically, it could be included in the fstab for fixed devices, such as floppy drives and CD/DVD drives. Removable devices are a little harder, as they may not have known names before hand. Auto mounting and things such as pmount handle them well and have options to set permissions, but how to get them to do this by default in all the environments is beyond me.
Doing this by default seems safe to me, but it would need more thought and consensus before change.
I should note that I'm not an Ubuntu dev, this is just my opinion on the matter.
Jesse